Known injection devices for internal combustion engines comprise an injector having a valve piston for controlling a flow of fuel and having a piezo actuator for moving the valve piston, and also a control unit for actuating the piezo actuator. In this case, the control unit is programmed to apply electrical pulses to the piezo actuator and therefore to cause the valve piston to move when a force which is exerted on the valve piston by the piezo actuator is greater than an opposing force. For interference-free operation of the internal combustion engine, it is necessary to control an injection instant and an injection duration, which are regulated by the movement of the valve piston, in a precisely timed manner.
Precisely timed control is inconsistent with, in particular, a so-called idle stroke between the piezo actuator and the valve piston. The idle stroke is defined as a distance through which the piezo actuator has to move or extend before it can move the valve piston. There is therefore a time delay between the actuation of the piezo actuator and the movement of the valve piston. In the present document, the term “idle stroke” is intended to be able to identify not only the length which corresponds to the idle stroke, but also another variable which is suitable as a measure for this idle stroke in the narrower sense of the word—that is to say for the said length—for example a charging quantity or an energy of an electrical pulse which allows the piezo actuator to overcome the idle stroke. However, on account of mechanical tolerances, temperature fluctuations, run-in behavior of a fuel and wear, an exact magnitude of the idle stroke is generally not known and furthermore subject to changes. In order to nevertheless be able to ensure interference-free operation of the internal combustion engine, it is therefore necessary to determine the idle stroke or the time delay which is caused by the idle stroke, and to take this into account when actuating the piezo actuator.
In a known method for determining the idle stroke, electrical pulses of different pulse energy are applied to the piezo actuator. In addition, a hydrostatic pressure is determined in a high-pressure store (rail), which supplies the injection device with fuel, by means of a pressure gauge before and after each of the pulses. If, in this case, the pulse energy of one of the pulses is so high that the piezo actuator overcomes the idle stroke and the opposing force, the piezo actuator moves the valve piston and causes a pressure drop in the rail. Knowing the pulse energy at which this pressure drop occurs allows the actuation of the piezo actuator to be adjusted during the operation of the internal combustion engine. One disadvantage of this method is that it cannot be carried out at any desired values of rotation speed, rail pressure or number of injection processes per second. If, in addition, a plurality of injectors are supplied with fuel by means of the same rail, a pressure drop in the rail cannot, under certain circumstances, be clearly attributed to an individual injector.
In another known method according to DE 10 2009 018 289 for determining the idle stroke, electrical pulses of different pulse energy are likewise applied to the piezo actuator. In addition, a voltage which is applied to the piezo actuator is determined after an end of a charging process and after a defined waiting time, the difference in said voltage being correlated with a change in force which is exerted on the piezo actuator. If the pulse energy is high enough to move the valve piston and as a result to cause a pressure drop in the rail, and a change in a force which is exerted on the piezo actuator by means of the valve piston accompanies the pressure drop, this change can be detected by means of the voltage across the piezo actuator. In this case too, knowing the pulse energy at which the change in force occurs allows the actuation of the piezo actuator to be adjusted during the operation of the internal combustion engine. One disadvantage of this method is that it cannot be used in valve pistons of so-called pressure-compensated valves in which the force which is exerted on the piezo actuator by means of the valve piston is independent of the rail pressure. However, valves specifically of this type are particularly cost-effective to operate since the piezo actuator requires little force to move these valves and as a result can be designed to be smaller.